Pulmonary hypertensive states are associated with an increased propensity for thrombosis. This prothrombotic state appears to be a result of pulmonary hypertension promoting endothelial dysfunction and altered hemodynamic status. In some patients with primary pulmonary hypertension, however, a primary prothrombotic state directly induces the pulmonary hypertensive state. This review focuses on the evidence for the association between prothrombotic states, especially increased platelet activation, and the development of pulmonary hypertension.